1. Field of the Invention
The present invention relates to attaching surface mountable components to both sides of a printed circuit card. More specifically, it relates to an improved method for cost effectively performing double sided assembly including two reflow operations without the need for using adhesive.
2. Description of the Prior Art
Commonly assigned U.S. Pat. No. 4,761,881 discloses a method for attaching surface mountable components to both sides of a printed circuit board and leaded components to a one side. Solder is screened onto surface mount lands and adhesive applied to the component receiving sites thereon and the components are placed. Adhesive ensures that the components remain in contact with the solder when the board is inverted. Solder is then screened into plated through-holes and onto surface mount lands on the second side of the board. Pin-in-hole and surface mount components are placed on the second side. Solder on both sides of the board is subjected to a single reflow operation.
U.S. Pat. No. 4,515,304 to Berger discloses a method for populating both sides of a printed circuit board and includes inserting the leads of leaded components through plated through-holes, cutting and clinching ends thereof prior to applying solder paste.
Research Disclosure, December, 1987, Issue No. 284, page 749 discloses a technique for enhanced double sided surface mount assembly which includes using a resist film as an adhesive.
JP 61056491 describes soldering both surfaces of a circuit substrate by mounting parts on the substrate surfaces followed by simultaneous dip and thermal emission reflow.
DE 3445625 describes soldering all components on both sides of a printed circuit board in one operation utilizing two different soldering methods. Components on one side of the board are held in position by adhesive. On the other side, a meltable solder is used to temporarily hold components. The assembly is then placed into a wave or dip solder machine.
Prior methods for double sided assembly usually require the use of adhesives. Several disadvantages are inherent in the use of adhesive. Adhesive restricts self centering and inhibits settling of components during reflow. Additional solder volume is required for filling gaps between leads and their associated pads.
Adhesive also causes difficulty during rework of printed circuit assemblies. Often tweezer type removal is part of a needed repair process. Adequate space within the card or board layout for accommodating twisting of the component to break the adhesive bond must be provided. Additionally, a problem may arise if adhesive remains on the circuit board and is not removed prior to component replacement. This adhesive residue must be removed before a successful reattach operation may occur.
When a single solder reflow process is used, the need for "no defect" operations is increased since extensive handling and cleaning may damage wet solder paste on the other side of the board.